Projection systems in many conventional projection apparatus comprise an LCD (liquid crystal display) to perform phase modulation or scattering modulation of the light.
New projection systems have been proposed that employ a DMD (digital micromirror device or deformable micromirror device) as the spatial light modulator. DMDs, as proposed in recent years, are disclosed, for example, Mignardi, "Digital Micromirror Array for Projection TV," Solid State Technology, July 1994, pp. 63-68; Sampsell, "An Overview of the Digital Micromirror Device (DMD) and Its Application to Projection Displays," SID 93 Digest, pp. 1012-1015; and Hornbeck, "Deformable-Mirror Spatial Light Modulators," SPIE Critical Reviews Series, Volume 1150, pp. 86-102, all of which references being incorporated herein by reference. DMDs typically comprise multiple micromirrors arrayed as a matrix on a substrate. DMDs are constructed so that the orientation of each micromirror is independently driven and controlled. The typical dimension of each micromirror is about 25 .mu.m.times.25 .mu.m, and one DMD can comprise, e.g., from several ten to several million micromirrors, depending upon application.
FIG. 1 is a schematic illustration, in a conventional projection system employing a DMD as a spatial light modulator, of general principles of operation of a micromirror. As discussed above, the DMD comprises a large number of individual micromirrors 22, each micromirror corresponding to an individual pixel of video data to be projected onto a screen (not shown) by a projection lens 21. The lens 21 is disposed a distance away from the micromirror array along an optical axis AX. Each micromirror 22 is illuminated with incident light impinging at an angle to the optical axis AX.
The angle of each micromirror is independently controlled. I.e., each micromirror is independently tiltable relative to the optical axis AX. At an appropriate tilt, as indicated by the dashed line in FIG. 1, light is reflected into the projection lens 21 and is directed onto a screen. Whenever the micromirror 22 is oriented perpendicularly to the optical axis AX, as indicated by the solid line in FIG. 1, light is reflected away from the lens 21 and does not reach the screen. Thus, a light/dark pattern of pixels can be formed on the screen as a video image according to the particularly tilt pattern of individual micromirrors in the DMD at a particular instant in time. Changing the tilt pattern as a function of time produces a moving image on the screen.
Thus, in a conventional projection system employing a DMD as a spatial light modulator, as described above, unwanted light is regularly reflected by certain micromirrors toward a location away from the aperture of the lens 21. Since the reflecting power of the micromirrors is extremely high, light reflected by the micromirrors away from the projection lens aperture has a high intensity and strong directivity. Unfortunately, it is difficult to prevent this reflected light from scattering or irregularly reflecting inside the optical system housing. Such stray light is a substantial factor in the production of "ghosts," which decrease the contrast of the image on the screen.